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Math Help - another normal distribution

  1. #1
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    another normal distribution

    X is a random variable following a normal distribution. Given the standard deviation is 12, and Pr(X < 25) = .15, find the mean.

    Hmm, any help?
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  2. #2
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    Quote Originally Posted by scorpion007 View Post
    X is a random variable following a normal distribution. Given the standard deviation is 12, and Pr(X < 25) = .15, find the mean.

    Hmm, any help?
    Consider Z=(X-mean(X))/12, then Z~N(0,1)

    So if:

    Pr(Z<A)=0.15

    we may look up the value of A which gives this probability. Doing this
    we find A=-1.03643, so the X=25 corresponds to A=-1.03643 or:

    -1.03643 = (25-mean(X))/12

    so:

    mean(X)= 25 + 12*1.03643~=37.44

    RonL
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  3. #3
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    thanks very much
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